Under normal circumstance, in order to meet the requirement of electromagnetic compatibility, a filter configuration comprising a capacitor is connected between AC power and the electronic devices. In compliance with the “safety of information technology equipment” standard, the capacitor is a safety capacitor which comprises X-capacitors and Y-capacitors. X-capacitor is connected between L line (Live line) and N line (Null line) of the input terminals of AC power.
Specifically positioned, X-capacitor will be charged upon the introduction of AC power, and will store energy in power plug when AC power is disconnected, which is likely to induce current leakage or make the enclosure charged to endanger user safety. Therefore, X-capacitor should be discharged rapidly in the event of AC power disconnection, and furthermore, the design of X-capacitor discharging circuit should meet related safety standard.
To comply with the “safety of information technology equipment” standard, one should ensure that the stored electric charge in a capacitor component of primary circuit is reduced to a minimal when designing electrical equipment at the external disconnection position of AC power. Specifically, if an electrical equipment is provided with a capacitor in the primary circuit, and if rated or nominal capacitance is more than 0.1 ii F and the discharge time constant of the capacitor is not more than the following specified values, the electrical equipment will be considered as qualified:                a) 1 second, for pluggable equipment of A type, and        b) 10 seconds, for equipment of permanent connection and pluggable equipment of B type;        
Wherein, the discharge time constant refers to a calculation from equivalent capacitance (μF) and equivalent discharge resistance (MΩ). That is to say, after a time period which equals to the discharge time constant, the voltage of the capacitor component will reduce to 37% of its initial value.
In usual ways, discharging resistor is used to connect in parallel with X-capacitor and the time constant of the capacitor and resistor should be less than the specified value to meet the rules concerning capacitor discharge in “safety of information technology equipment”. Next, the X-capacitor discharging technology in the prior art will be described using an example that an X-capacitor is comprised in a converter circuit.
FIG. 1 shows a circuit schematic diagram of a converter with an X-capacitor in the prior art, in which a discharging resistor is connected in parallel between the two terminals of the X-capacitor. As shown in the figure, an X-capacitor 2, a discharging resistor 3 and a converter module 1 are coupled successively, and the X-capacitor 2 is connected in parallel between the input terminals (between L line and N line) of AC power. The converter with the X-capacitor in the prior art is the same as other electrical equipment in which a discharging resistor is connected in parallel between the two terminals of the X-capacitor, i.e. high voltage energy is stored in the X-capacitor 2 for long time in the event of AC power disconnection, the discharging resistor 3 is used to provide discharging path for the X-capacitor 2 to meet the safety requirement.
However, the discharging resistor 3 will consume energy all along and induce power loss, especially, the power loss will become greater when the input voltage being higher, which is an important factor leading to the power loss of the converter during standby mode and no-load mode. With the increasing requirement of efficiency at the light-load mode, it is more and more important to minimize the power loss caused by the discharge of the X-capacitor 2.
Furthermore, the converter module 1 can be a two-stage converter composed of a power factor correction (PFC) conversion unit and a DC/DC conversion unit, and also can be a single-stage conversion module. If the converter module is the two-stage converter, the PFC conversion unit thereof can be a bridge PFC conversion unit or a bridgeless PFC conversion unit, and it can also be a boost PFC conversion unit, a buck PFC conversion unit or a boost-buck PFC conversion unit.
It should be stressed that the two-stage conversion module having a bridgeless PFC conversion unit has much higher efficiency in the case of heavy load due to the advantage of topology, compared to the conventional PFC conversion unit having a bridge structure, but the X-capacitor having greater capacitance needs to be used. If the discharging resistor is used to discharge the X-capacitor, the resistor having relative low resistance is preferred. The lower the resistance is, the greater the power loss of the discharging resistor is when AC power is introduced, which could lead to lower efficiency of the bridgeless PFC conversion unit in the light-load mode. Therefore, it becomes more and more urgent to solve the problem of the power loss induced by discharge of the X-capacitor.
Consequently, to meet the requirement of “safety of information technology equipment” and meanwhile to achieve high efficiency of electrical equipment especially in the light-load mode is becoming a pressing problem needs to be resolved.